1. Field of the Invention
The present invention relates to a sheet member holding apparatus wherein a pair of arms, which extend in opposite peripheral directions from a fulcrum attached to a part of a peripheral surface of a rotary body, are pivoted about the fulcrum by a centrifugal force generated by rotation of the rotary body, one of the arms approaches the peripheral surface of the rotary body, and a sheet member wound around the peripheral surface of the rotary body is held by a clamp section attached to the one of the arms.
2. Description of the Related Art
For printing, a photosensitive printing plate (hereinafter, “printing plate”), in which a photosensitive layer is formed on a sheet-type supporting body such as a thin plate made of aluminum, is generally used. As such printing plates, printing plates having different longitudinal and breadthwise dimensions according to printing size are used.
An image exposure apparatus for performing image exposure on a printing plate includes an apparatus which winds a printing plate around a rotary drum and then emits a light beam based on image data to the printing plate while rotating the printing plate integrally with the rotary drum, so as to scan-expose the printing plate.
When the printing plate is wound around the rotary drum, the printing plate is nipped and fixed to the rotary drum by holding apparatuses (chucks) for clamping both ends, in the peripheral direction of the rotary drum, of the printing plate (nipping the plate between the chucks and the peripheral surface of the rotary drum).
That is, a holding apparatus corresponding to one end of the printing plate along the drum peripheral direction (for example, the end at a winding front end side) is attached to a predetermined position of the rotary drum, and, after the printing plate has been wound around the rotary drum, a holding apparatus corresponding to the other end of the printing plate (here, the end at a winding rear end) is attached to a position according to the size of the printing plate.
Here, the holding apparatus for attaching the printing plate to the rotary drum at the position according to the size has a fixing barrel (stanchion) which can be inserted into an arbitrary position of an attachment groove which is formed along the peripheral direction in the peripheral surface of the rotary drum. The fixing barrel attached to the holding apparatus is freely movable relative to the attachment groove and can nip the ends of printing plates having various sizes in suitable positions.
When the printing plate wound around the rotary drum is to be exposed, the printing plate is closely contacted with the peripheral surface of the rotary drum and rotated at high speed together with the rotary drum.
At this time, in order to prevent lifting of the end of the printing plate fixed by the clamp section of the holding apparatus due to centrifugal force, a structure in which the centrifugal force is utilized to instead increase holding force of the clamp section has been suggested (for example, see Japanese Patent Application Laid-Open No. 2000-112142).
In this publication (of the prior art), because force is applied by the centrifugal force to portions to be held, a ratio of lengths from a fulcrum to the ends of a seesaw-type arm (a mechanism where a center portion is the fulcrum and the arm pivots about the fulcrum) is prescribed, and a strong holding force is applied at a distal end of the arm at a clamp section side thereof. In this prior art, a combined structure in which the sheet material is adsorbed to the rotary drum when wound around the rotary drum is also used.
However, in the above prior art, holding of the sheet material is mainly due to the adsorption, and holding reinforcement due to the centrifugal force is utilized supplementarily. Moreover, the publication discloses only prescription of the lengths of the arms and simply describes the well-known law of moment. Therefore, a position of the center of gravity in the holding apparatus, a coefficient of friction between the holding apparatus and the sheet material at the time of holding the sheet material, a coefficient of friction between the sheet material and the rotary drum, and the like are not taken into consideration.
However, in a case where the sheet material is not adsorbed to the rotary drum, the above coefficients of friction and the like can be important conditions, and the centrifugal force cannot be utilized effectively merely by prescribing the lengths of the arms.